Doxylamine succinate is one of the many sedating antihistamines used by itself as a short-term sleep aid, in combination with other drugs as a night-time cold and allergy relief drug, and a preparation is prescribed in combination with Vitamin B6 (pyridoxine) to prevent morning sickness in pregnant women. It is one of the active ingredients in NyQuil. In Commonwealth countries, such as Australia, Canada, the United Kingdom, doxylamine is available prepared with paracetamol (acetaminophen) and codeine under the brand name Syndol or Mersyndol, as treatment for tension headache and other types of pain, or as a general over-the-counter sleep-aid branded as Somnil, Donormyl, DSozile, Restavit and Unisom-2 containing doxylamine succinate only. In the United States doxylamine succinate is the active ingredient in the over-the-counter sleep-aid tablets branded as Unisom.